A team of international scientists, led by those from India, has found a distinctive signature of cosmic X-rays to identify the boundary around black holes, which “unmistakably separate them” from other objects in the cosmos such as neutron stars that are comparable in mass and size. According to the astrophysicists, including Sudip Bhattacharyya from the Tata Institute of Fundamental Research (TIFR) in Mumbai, the current discovery is by far the strongest steady signature of the smaller, but more extreme stellar-mass black holes to date, from the cosmic X-rays observed with a satellite.
 
 
What
  1. Although black holes do not have a surface, it is confined within an invisible boundary, called an event horizon, from within which nothing, not even light, can escape.
  2. While definitive proof of the existence of such objects is a holy grail of modern physics and astronomy, they said only one supermassive black hole with the mass more than six billion times the mass of the Sun has so far been imaged using the surrounding radiation in radio wavelengths.
  3. In order to prove the existence of these stellar-mass black holes, the researchers said these need to be distinguished from neutron stars which are the densest known objects in the universe with a hard surface.
  4. While the stable stellar-mass black holes shine mainly in X-rays by devouring material from a companion star, the study noted that neutron stars can also shine in X-rays by accreting matter from a companion star in a similar way.
  5. In the current study, the scientists analysed archival data from the now decommissioned astronomy satellite Rossi X-Ray Timing Explorer, and have identified the effect of the lack of hard surface in black holes on their observed X-ray emission.
  6. From these analyses, they have found an extremely strong signature of accreting stellar-mass black holes.
  7. The study has found by far the strongest steady signature of smaller, but more extreme, black holes to date, from the cosmic X-rays observed with a satellite.
  8. A neutron star, the densest known object in the universe with a hard surface, can also shine in X-rays by accreting matter from a companion star in a similar way, characterized by extremely high efficiency of conversion of the rest-mass energy mc2 to radiation, of the order of 20%. 
  9. In order to prove the existence of stellar-mass black holes, one needs to distinguish them from such neutron stars.